1. Field of the Invention
The present invention relates to an image processor being incorporated, for example, in a digital copier, and outputting input image information after processing it, an image processing method thereof, and a storage medium in which a program therefor is stored.
2. Description of the Related Art
In an image processor being incorporated, for example, in a digital copier, and outputting image information input from an image input device such as a scanner or a personal computer after processing it, an image processing method thereof and a storage medium in which a program therefor is stored, generally, pseudo tone processing is performed in order to reduce the load on the memory and increase the data processing speed. Conventionally, as such a pseudo tone processing, multilevel error diffusion processing is well known in which analog continuous tone input data is replaced with predetermined number of gradations by quantizing it into absolute tone data distributed at fixed tone differences, thereby reducing the amount of data to be processed.
In the previously known multilevel error diffusion processing, normally, the error diffusion processing is performed on pixel data constituting an image and successively input from an image input device irrespective of the tone level. Moreover, in the conventional multilevel error diffusion processing, random noise or dither noise is added to the threshold value level in error detection. According to such a processing method, the result of the pseudo tone processing is not excellent in a highlighted area and a high-density area, character edge jaggies are noticeable and the result of pseudo tone is not excellent in color superimposition.